electromagentic theory i - cern · for conservative field v(p) does not depend on the path ! ......
TRANSCRIPT
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Electromagnetic Theory
G. Franchetti, GSI
CERN Accelerator – School
Budapest, 2-14 / 10 / 2016
3/10/16 G. Franchetti 1
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Mathematics of EM
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Fields are 3 dimensional vectors dependent of their spatial position (and depending on time)
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Products
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Scalar product Vector product
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The gradient operator
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Is an operator that transform space dependent scalar in vector
Example: given
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Divergence / Curl of a vector field
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Divergence of vector field
Curl of vector field
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Relations
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Flux Concept
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Example with water
v
v
v
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Volume per secondor
L
θ
v v
v
v
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Flux
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θ
A
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Flux through a surface
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Flux through a closed surface: Gauss theorem
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Any volume can be decomposed in small cubes
Flux through a closed surface
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Stokes theorem
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x
y
Ey(D,0,0)
Ey(0,0,0)
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for an arbitrary surface
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How it works
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Electric Charges and Forces
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+ -
Two charges
Experimental facts
+ +
+ -
--
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Coulomb law
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Units
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System SI
Newton
Coulomb
Meters
C2 N-1 m-2
permettivity of free space
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Superposition principle
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Electric Field
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3/10/16 G. Franchetti 20
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By knowing the electric field the force on a charge is completely known
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Work done along a path
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work done by the charge
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Electric potential
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For conservative field V(P) does not depend on the path !
Central forces are conservative
UNITS: Joule / Coulomb = Volt
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Work done along a path
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work done by the charge
A
B
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Electric Field Electric Potential
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In vectorial notation
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Electric potential by one charge
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Take one particle located at the origin, then
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Electric Potential of an arbitrary distribution
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set of N particles
origin
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Electric potential of a continuous distribution
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Split the continuous distribution in a grid
origin
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Energy of a charge distribution
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it is the work necessary to bring the charge distribution from infinity
More simply
More simply
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In integral form
Using and the “divergence theorem” it can be proved that
is the density of energy of the electricfield
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Flux of the electric field
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θ
A
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Flux of electric field through a surface
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Application to Coulomb law
On a sphere
This result is general and applies to any closed surface
(how?)
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On an arbitrary closed curve
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q1
q2
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First Maxwell Law
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integral form
for a infinitesimalsmall volume
differential form
(try to derive it. Hint: used Gauss theorem)
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Physical meaning
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If there is a charge in one place, the electric flux is different than zero
One charge create an electric flux.
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Poisson and Laplace Equations
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As and
combining both we find
In vacuum:
Poisson
Laplace
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Magnetic Field
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There exist not a magnetic charge! (Find a magnetic monopole and you get the Nobel Prize)
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Ampere’s experiment
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I1
I2
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Ampere’s Law
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I
I2
F
all experimental results fit with this law
B
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Units
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From [Tesla]
From follows
To have 1T at 10 cm with one cable
I = 5 x 105 Amperes !!
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Biot-Savart Law
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I
From analogy with the electric field
dB
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Lorentz force
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A charge not in motion does not experience a force !
v
B
F
No acceleration using magnetic field !
+
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Flux of magnetic field
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There exist not a magnetic charge ! No matter what you do..
The magnetic flux is always zero!
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Second Maxwell Law
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Integral form
Differential from
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Changing the magnetic Flux...
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h
L
Magnetic flux
Following the path
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Faraday’s Law
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integral form
valid in any way the magnetic flux is changed !!!
(Really not obvious !!)
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for an arbitrary surface
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Faraday’s Law in differential form
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Summary Faraday’s Law
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Integral form
differential form
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Important consequence
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A current creates magnetic field
magnetic field create magnetic flux
Changing the magnetic flux creates an induced emf
L = inductance [Henry]
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energy necessary to create the magnetic field
h
r
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Therefore
Energy densityof the magneticfield
Field inside the solenoid
Magnetic flux
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Ampere’s Law
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I
B
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Displacement Current
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I
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Displacement Current
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Here there is a varying electric field but no current !
I
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Displacement Current
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Stationary current I electricfield changes with time
I
This displacement current has to be added in the Ampere law
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Final form of the Ampere law
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integral form
differential form
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Maxwell Equations in vacuum
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Integral form Differential form
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Magnetic potential ?
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Can we find a “potential” such that
Maxwell equation
Butit means that we cannotinclude currents !!
?
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Example: 2D multipoles
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For 2D static magnetic field in vacuum (only Bx, By)
These are the Cauchy-ReimannThat makes the function
analytic
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Vector Potential
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In general we require
(this choice is always possible)
Automatically
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Solution
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Electric potential Magnetic potential
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Effect of matter
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Electric field Magnetic field
Conductors
Dielectric
DiamagnetismParamagnetismFerrimagnetism
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Maxwell equation in vacuum are always valid, even when we consider the effect of matter
Microscopic field
That is the field is “local” between atoms and moving charges
Averaged field
this is a field averaged over a volume that contain many atoms or molecules
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Conductors
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free electrons bounded to be inside the conductor
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Conductors and electric field
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surface distribution of electrons
bounded to be inside the conductor
on the surface the electric field is always perpendicular
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Applying Gauss theorem
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Boundary condition
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The surfaces of metals are always equipotential
++
+
--
--
+
+ +
++
-
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Ohm’s Law
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free electrons
l
A
[Ω]
resistivity
or
conductivity
[Ωm]
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Who is who ?